1. Technical Field of the Invention
The present invention relates to a semiconductor photo-detector and its manufacturing method, and particularly, to a small-sized planar semiconductor photo-detector with high conversion efficiency.
2. Description of the Prior Art
Conventionally, light absorption materials for photodiodes used in communication technologies are different depending upon wavelengths to be detected. In general, the narrower is the band gaps of the light absorption materials, the higher is the detection sensitivities. Accordingly, Si is used for the wavelength range below 1.0 .mu.m, while Ge, or InGaAs is used for longer wavelength region over 1.0 .mu.m.
The materials such as Ge for longer wavelength, in comparison with Si, are expensive, not easy in processing, and difficult to integrate it with other circuit elements. So, there have been expectations for a long time for photodiodes which are highly sensitive for longer wavelength and is produced by using Si processes which can save the production cost.
Further, the materials such as Ge crystal are hardly grown on a Si substrate, because their lattice constants are different from Si. Therefore, Ge can not be deposited directly on the Si substrate. Accordingly, a photodiode as disclosed in, for example, B. Jalali et al., Journal of Lightwave Technology, Vol. 12, June 1994 pp 930-935, includes light absorption layer 22 on Si substrate 20, as shown in FIG. 7A, wherein SiGe mixed crystal and Si are deposited alternatively.
Si single crystal layer 21 and P-type-silicon layer 23 are shown in FIG. 7A. The band gap in a Si photo detecting portion doped by Ge becomes narrower than that of Si.
Therefore, the photodetector as shown in FIG. 7A has some detection sensitivity in the longer wavelength, wherein Si and Si/Ge are deposited alternately to obtain a sufficiently thick SiGe film, because SiGe can not be deposited thick directly on the Si substrate, due to the differences of both lattice constants.
The inventors of the present invention discloses a planar SiGe photodiode, wherein the Si/SiGe layer is buried in a Si substrate, in JP 07-231113A (1995), and JP08-316449A (1996). Further, the fabrication result is reported in M. Sugiura at al., International Electron Device Meeting 1995 Technical Digest pp583-586.
FIG. 7B is a cross sectional view of the planar SiGe photodiode. As shown in FIG. 7B, Si/SiGe light absorption layer 22 is buried in a trench whose wall is covered by silicon dioxide 27. Here, the silicon dioxide wall is a mask used for a selective epitaxy for Si/SiGe. In FIG. 7B, there are shown Si dioxide film 25, N-buried layer 28, N-epitaxial layer 26, and P-type-Si layer.
Thus, Silicon Opto-Electric Integrated Circuit (Si-OEIC) for longer wavelength can be easily fabricated, because Si transister integrated circuit and photodiode can be formed on a single Si substrate. On the other hand, as shown in FIG. 8, a Ge crystalline light absorption layer on Si with an interface layer between Si and Ge is disclosed in JP 61-500466A (1986).
As shown in FIG. 8, SiGe layer 30 is deposited on Si substrate 29, wherein the Ge content is gradually increased to 100% at the top surface of the film. In FIG. 8, there are also shown N-type-Ge layer 31, Ge single crystal layer 32, and P-type-Ge layer 33.
Ge single crystal layer 32 can be grown, without depending upon the differences of lattice constants. Further, intermediate SiGe layer 30 can be deposited, because Ge content is gradually changed. Furthermore, as shown in FIG. 9, a method of direct Ge crystal growth on Si substrate and its application to photodiodes are disclosed in JP09-70933A (1997).
As shown in FIG. 9, an extremely thin Si or SiGe layer is grown on a thin Ge layer deposited on a Si substrate. Then, dislocations are localized in the interface between the two films to banish the dislocations from the Ge layer, by a thermal treatment. Therefore, Ge single crystal can be grown up to a desired thickness, owing to the lattice relaxation in the thermally treated thin Ge film.
As shown in FIG. 9, Ge single crystal layer 32 is grown directly on Si substrate 29 to fabricate a photodiode by selective epitaxy by using oxidized silicon as a mask. Further, there is shown in FIG. 9 a planar waveguide photodiode for longer wavelengths, wherein an optical fiber 34 is placed on groove 26 which is formed on Si substrate 29.
As shown in FIG. 10, photodiode 36 is made slightly wider than the core of optical fiber 34, taking the divergence of light beam into consideration in case of using a waveguide photodiode which accepts light beam in lateral direction as shown in FIG. 9.
However, as the width of photodiode 36 increases, the area of the PN junction increases, which results in a degradation in high frequency characteristics due to the increase in junction capacity. Therefore, a fan-shaped photodiode as shown in a plan view of FIG. 11 is disclosed in JP 08-316449A (1997), in order to suppress the junction capacity.
However, the junction capacity in the photodiode as shown in FIG. 11 is inevitably increased, although the junction area is suppressed in some degree without decreasing the quantum efficiency. On the other hand, when the width of a photodiode is not made wider, the coupling efficiency becomes lower, because the light beam diverges laterally, as shown in FIG. 10.
Any groove made by reflective material such as silicon dioxide in order to prevent the light beam from diverging can not be provided along the light path, because the lower electrode of the photodiode can not be extracted over the surface of the substrate. Therefore, the absorption efficiency for the diverging beam can not be maximized in the conventional semiconductor photo-detector, when its size is minimized.
Any technology for minimizing the size of photo-detector is not disclosed in JP 08-330671A (1996), although it is disclosed that a lateral width of an optical wave -guide is modulated.